Doves, crows and other birds incur damages by their droppings. Dropping damages sometimes pose a serious trouble. In a huge building such as an aeroplane hangar, a large-scale warehouse, or a big plant, its entrance and exit have to be left open because of frequent incoming and outgoing operations of planes and transport vehicles. A bird easily enters it. Birds having a nest in the building have generally a strong attachment to it, and doves are particularly notorious for it. Even if they are frightened away, they will definitely come back again and there was no effective solution to it.
Natural enemy relationships are established among birds. For example, birds of prey such as eagles, hawks and falcons are thought of as the natural enemies of doves and crows. When the dove and crow recognize the presence of such a natural enemy nearby, they immediately fly away. If the dove and crow encounter the enemy repeatedly, they will never access the same location.